add general guideline for technical blog posts

perhaps in https://iceberg.apache.org/community/#community-guidelines
perhaps also include in AGENTS.md

Generally we want the technical blogs to stay technical and not endorse any specific projects, products, companies, etc.
Litmus test:
- "Could this be used as an advertisement for a project"
- "This is only interesting to users and devs"

Read the community guidelines at https://iceberg.apache.org/community/#community-guidelines and check AGENTS.md to determine where contribution guidance belongs. Add a general guideline stating that technical blog posts should remain technical and not endorse projects, products, or companies, using the two listed litmus tests to define the boundary.

Written by the indexing model from the issue text.